Flame suppressants can be classified as either active (chemical) or passive (physical) suppressants. Active suppressants or suppression agents, also called “fire retardants,” react chemically with and destroy free radicals in the flame. Free radicals are very short-lived species that catalyze flame reactions, and their chemical removal or modification in turn retards the flame. Passive suppressants, also called “fire suppressants,” often seek to deprive the combusting fuel from oxygen by physically interfering with its transport or access to the flame combusting fuel.
One form of active suppressant is a class of materials sold as Halon™ which are composed of brominated or chlorinated fluorocarbon compounds, e.g., bromochlorodifluoromethane (CF2BrCl) and trifluorobromomethane (CF3Br). These and competitive materials exhibiting similar chemistry have been used effectively as fire suppression agents for years, typically to protect electrical equipment since there is very little residue to clean up. These fire suppression agents typically interrupt the chemical reaction that takes place when fuels burn and depend on a combination of chemical effectiveness, e.g., quenching of free radicals, and some physical effectiveness, e.g., cooling the combustion flame and dilution of the combustion ingredients. Certain halogen-containing fire suppression agents, however, such as CF3Br, contribute to the destruction of stratospheric ozone. Although the materials are essentially nontoxic, passage through a flame or over hot surfaces can produces toxic fluorine compounds.
To reduce the environmental effects associated with halogenated fluorocarbons, many commercially available fire suppression agents designed today are passive, i.e., physically acting, agents. A passive suppressant does not react chemically with the flame. These fire suppression agents either blanket the burning material to deprive it of oxygen, dilute the oxygen in the environment to below the point that can sustain the flame, or cool the burning surface below its ignition temperature. Examples of physically-acting fire suppression agents include sodium bicarbonate and sand as well as inert gases, e.g., carbon dioxide (CO2), water vapor (H2O) and nitrogen (N2). When applied to a fire, inert gases physically displace oxygen from the combustion region while simultaneously serving as a heat sink to reduce the temperature of the flame. The combination of the two physical actions results in suppression of the fire.
Physically-acting fire suppression agents are, however, also subject to certain issues and problems that can reduce their effectiveness at fire suppression. The agents typically require, for example, a large quantity of a physically-acting fire suppressant in order to suppress a fire and, consequently, equipment and storage must be correspondingly large to accommodate the required quantities. Such large equipment is a disadvantage in limited spaces and where additional equipment weight is an issue such as in aircraft and spacecraft. Another disadvantage of physical suppressants is they must often be applied directly to a combusting surface, which can inhibit their effectiveness against fires that are concealed or relatively inaccessible.